A “data leak prevention” (DLP) system is a technology designed to keep a company's data in that company's network, or at least so the company can manage it and influence its motion on the company's network. A DLP assumes that it knows where the data is which it is monitoring. Classic examples of problems the DLP is trying to solve are people e-mailing to places where they shouldn't or with attachments that they shouldn't, people trying to e-mail out the company's source code, people trying to download the company's credit card database, and the like.
Initially DLP was positioned as something designed to stop malicious users from exfiltrating data out of the network. In practice, it turned into something to keep people from doing stupid things with data. It turned into a stupidity prevention system. Basically, a DLP was good for things like preventing fat-fingering auto-complete e-mail addresses, for example, when the company's provisional quarterly numbers are accidentally sent to the Washington Post instead of the company's accountant.